The metabolic capabilities of animals have been derived from
well-studied model organisms and are generally considered to be well
understood. In animals, cysteine is an important amino acid thought to
be exclusively synthesized through the transsulfuration pathway. Corals
of the genus Acropora have lost cystathionine β-synthase, a key enzyme of the transsulfuration pathway, and it was proposed that Acropora
relies on the symbiosis with dinoflagellates of the family
Symbiodiniaceae for the acquisition of cysteine. Here, we identify the
existence of an alternative pathway for cysteine biosynthesis in animals
through the analysis of the genome of the coral Acropora loripes.
We demonstrate that these coral proteins are functional and synthesize
cysteine in vivo, exhibiting previously unrecognized metabolic
capabilities of animals. This pathway is also present in most animals
but absent in mammals, arthropods, and nematodes, precisely the groups
where most of the animal model organisms belong to, highlighting the
risks of generalizing findings from model organisms.